Conventional hybrid electric vehicles combine internal combustion (IC) engines with electric propulsion systems to achieve better fuel economy than non-hybrid vehicles. Plugin hybrid electric vehicles (PHEVs) share the characteristics of both conventional hybrid electric vehicles and all-electric vehicles by using rechargeable batteries that can be restored to full charge by connecting (e.g. via a plug) to an external electric power source.
PHEVs are generally designed to operate in two modes. In the first mode, known as a charge depleting mode, vehicle operation results in a net discharge of the battery. Most PHEVs operate only the electric propulsion system in this first, charge depleting mode so as to provide all-electric operation. Charge depleting mode operation is generally continued until the battery charge drops below a threshold, at which time the PHEV operates in a second, charge sustaining mode. In the charge sustaining mode, the PHEV operates much like a conventional hybrid, using both the IC engine and electric propulsion system to keep the battery at a sufficiently high level of charge to enable, for instance, start/stop operation of the IC engine.
Generally, powertrain control in PHEVs is designed such that the PHEV aggressively operates in charge depleting mode from the beginning of each drive cycle. Charge depleting mode remains active until the battery charge level drops below a charge sustaining threshold. After this, the PHEV operates in charge sustaining mode. Thus, the IC engine, if and when it operates, will not likely be operated as efficiently as it could. Likewise, because the PHEV generally uses the electric propulsion system exclusively in the charge depleting mode, it too will likely not be operated as efficiently as it could. In a typical PHEV, the power needs of the vehicle are not known for any drive cycles, even if the vehicle is travelling a highly repeated route.